Effect of Nonselective His Bundle Pacing on Delayed Myocardial Activation in Left-axis Deviation and Left Bundle Branch Block

Figures

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  Figure 1:. Effect of NS-HBP on isolated LAD. Twelve-lead ECG panel, 25 mm/s. The transition from NS-HBP to S-HBP is shown, marked by an asterisk. Note the fall in L1V with S-HBP. A shift in the QRS axis occurred from 5° to −30° and late transition is seen in V leads.

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  Figure 2:. Comparison of baseline LBBB with NS-HBP with and without a notch and S-HBP. Recordings in the figure were made at 100 mm/s. A: At baseline LBBB, the H–V interval was 88 ms, the interval from the His bundle electrogram to the septal notch was 148 ms, and the interval from the His bundle electrogram to the lateral wall was 189 ms. The L1V was 0.24 mV. The V lead transition occurred in V6. B: NS-HBP complex (first and third beats) without evidence of septal notch. The stimulus to lateral wall interval was reduced by 64 ms to 125 ms, L1V increased to 0.67 V, and the QRS transition is now in V4. As the pacing voltage is reduced, the second beat shows a wide complex with a mid-QRS notch in V6; however, the stimulus to septal notch interval of 80 ms and the stimulus to lateral wall interval of 121 ms were less than those in LBBB. The L1V is higher than in LBBB and less than in NS-HBP, and the QRS axis is normal. C: The transition from NS-HBP to S-HBP and complete LBBB are seen as the paced impulse now conducts exclusively via the His bundle and blocks in the left bundle branch. HVAT: His Purkinje system ventricular-activation time.

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  Figure 3:. NS-HBP and S-HBP in a patient with rate-related LBBB. Recordings in the figure are presented at 100 mm/s. A: A sinus beat with an H–V interval of 43 ms, a His to peak QRS interval of 120 ms, and a QRS interval of 114 ms. B: Atrial pacing at 120 bpm resulted in rate-related LBBB. Note the notched wide QRS, decrease in L1V, leftward QRS axis, and loss of R-waves in the chest leads. C: HBP at 130 bpm. NS-HBP (first two beats) shows the paced complex to be very similar to the native QRS interval in A without LBBB; as the pacing voltage is reduced from 2.0 to 1.8 V, the third beat shows widening of the QRS interval followed by transition from NS-HBP to S-HBP. The “delta” wave is prolonged (third beat) and lost in the next beat as conduction block occurs in the NS-HBP pathway. In the last two beats, S-HBP is seen with uncorrected LBBB, which is identical to the rate-related LBBB in B. D: Normal NS-HBP at 140 bpm. Note that the delta waves in the NS-HBP complex are subtle and seen in the initial portion of leads 2, 3, and AVF.

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  Figure 4:. Comparison of LBBB and NS-HBP in iatrogenic complete heart block. A: LBBB with LAD (at 100 mm/s). B: Complete heart block, which occurred during active fixation of the His bundle lead (at 25 mm/s to allow for more complexes to be seen). RV paced complexes (first two beats) can be compared to NS-HBP complexes (third and fourth beats). C: The complex is likely caused exclusively by NS-HBP given the complete heart block. As compared with LBBB, the NS-HBP complex has a narrow QRS, a higher L1 voltage, a normal frontal QRS axis, and R-waves in the anterior chest leads.

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  Figure 5:. NS-HBP showing fusion with the underlying LBBB. Recordings in the figure are presented at 100 mm/s. A: LBBB with a His to septum interval of 131 ms and a His to lateral wall interval of 193 ms. B: NS-HBP with mid-QRS notching; however, the stimulus to septum interval was 65 ms and the stimulus to lateral wall interval was 99 ms, which are significantly shorter than in LBBB. Also, L1V is higher and the QRS axis is normal, with a more normal R-wave progression.

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  Figure 6:. Comparison of excitation pathways in normal activation and in NS-HBP. A: A transverse section of the heart with blue arrows representing the normal excitation of the heart.15 The left anterior basal paraseptal (LABP) segment, the distal third of the left septum, and the right posterior paraseptal area (RPPA) are the first to be excited.15 Also depicted is the normal left to right septal activation. In LBBB, slow septal activation would occur from the right to the left, starting from RPPA, which is the exit point of the right bundle branch.12–14 LAD occurs due to delayed activation of the LABP segment. B: The recently described specialized conducting cardiomyocytes (SCCs) (shown as pale striations), which make up the distal conduction network.19–21 C: A likely activation sequence in NS-HBP. The LAD is corrected by early excitation of the LABP segment, while a higher L1V may be explained by the right to left septal activation; however, shorter QRS activation times would require utilization of the specialized conduction network. LV: left ventricle; RV: right ventricle.